(1) Field of the Invention
The present invention relates a sensor driving/measuring system for driving a sensor and measuring an output of the sensor.
(2) Disclosure of Related Art
Conventional sensor driving/measuring systems are generally equipped with semiconductor integrated circuits for driving and measuring sensors.
It is necessary to supply stable power supply voltages to A/D converters that convert analog signals, such as sensor outputs, into digital signals and process the resultant signals. The voltage ranges of measurable signals are limited by the power supply voltages of the A/D converters. Specifically, a high signal voltage which is to be measured requires a high power supply voltage.
In battery-driven equipment, the electromotive forces of batteries differ between new batteries and batteries in which only small amounts of charge remain. To extend the operation time of the equipment, stable operation even at power supply voltages of such batteries with small amounts of remaining charge is demanded.
To meet this demand, a conventional sensor driving/measuring system driven by batteries and using a semiconductor integrated circuit has a configuration as illustrated in FIG. 4. or 5. The configuration illustrated in FIG. 4 employs a semiconductor integrated circuit 402 that has a high breakdown voltage, is used for driving/measuring a sensor, and is operable in a wide power-supply-voltage range from the voltage of a new battery to the electromotive force of a battery with a small amount of remaining charge. It should be noted that batteries 404 need to have at least an electromotive force enough to allow the semiconductor integrated circuit 402 to apply, to a sensor 403, a voltage required by the sensor 403. In the configuration illustrated in FIG. 5, a semiconductor integrated circuit 502 with an ordinary breakdown voltage is connected to batteries 504 through a regulator IC 508 that generates a stable voltage in a wide power-supply-voltage range from the voltage of a new battery to the electromotive force of a battery with a small amount of remaining charge. It should be noted that the batteries 504 need to have at least an electromotive force enough to allow the semiconductor integrated circuit 502 to apply, to a sensor 503, a voltage required by the sensor 503.
In the structures of the battery-driven sensor driving/measuring systems illustrated in FIGS. 4 and 5, the power supply voltages need to be monitored and managed so that the amounts of charge remaining in the batteries 404 and 504 are determined. The configuration illustrated in FIG. 5 also needs external parts, such as a switch 509 for stopping the flow of current when no power supply voltage is detected and a voltage dividing resistance 510, in order to prevent the power supply voltage from being directly applied to the semiconductor integrated circuit 502 and to suppress unnecessary current consumption.
In the field of A/D converters, the technique of switching from a power supply voltage to an A/D converter to a boosted voltage when the power supply voltage is low is known, for example (see, for example, Patent Document 1).
In the field of boosters, the technique of switching an output current of a regulator serving as a load of the booster to prevent a drop of a boosted voltage is known, for example (see, for example, Patent Document 2).
(Patent Document 1)
Japanese Unexamined Patent Publication No. 2003-179492
(Patent Document 2)
Japanese Unexamined Patent Publication No. 2005-44203
For semiconductor integrated circuits, voltages at which the circuits normally operate and voltages at which breakdown of the circuits does not occur are generally determined according to fabrication processes. As the upper limits of these voltages become higher, the cost increases. Specifically, the fabrication process becomes more complicated and the size of each element increases, for example.
On the other hand, for sensor driving/measuring systems, voltages applied to drive sensors and voltages at points where sensor current to be measured is detected are determined according to characteristics and configurations of the sensors in most cases. In other words, such voltage specifications are not determined to suit the convenience of the sensor driving/measuring systems and, therefore, sensor driving/measuring systems including semiconductor integrated circuits are often designed according to sensor characteristics.
When a voltage applied to a sensor is high, the power supply voltage needs to be high. Accordingly, for example, the following measures are taken: a sensor driving/measuring semiconductor integrated circuit having a higher breakdown voltage is designed by increasing the number of incorporated batteries or by using batteries having higher electromotive forces; and a sensor driving/measuring semiconductor integrated circuit is used together with a regulator IC which absorbs variations in the power supply voltage. In either case, increase of the cost is inevitable in current situations.